Self-sensing refers to the ability of a material, typically a structural material, to sense its own condition. Strain sensing is a key function in structural control and numerous types of strain sensor are available, including optical fibres, piezoelectric sensors, electrostrictive sensors, magnetostrictive sensors and piezoresistive sensors.
Composite materials including fibre reinforcements, such as carbon fibre reinforced polymers (CFRP) and glass fibre reinforced polymers (GFRP), have become relatively commonly-used structural materials. Among the various types of fibre proposed for this purpose, carbon fibre in particular has become increasingly dominant, due to its high strength, high modulus, low density, and temperature resistance. As a result, CFRP composite materials are gaining wider use across a number of industries.
Carbon fibres are electrically conductive, and a change in their electrical resistance occurs in response to strain. It has, therefore, been proposed to use self-sensing strain measurement in carbon fibre reinforced materials by exploiting the strain/resistance response of carbon fibres, thereby eliminating the need for conventional embedded or attached sensors for the purpose of structural control, and resulting in reduced cost, greater durability, larger sensing volume, and absence of mechanical property degradation (due to embedded sensors).
Thus, referring to FIG. 1 of the drawings, a typical example of a fibre reinforced polymer material 100 comprises a resin substrate 102 having embedded therein a plurality of elongate carbon fibres or carbon fibre ‘tows’ 104, which may be arranged in substantially parallel, side-by-side configuration across the principal plane of the material, and in several different alternative orientations and configurations, as illustrated in FIGS. 1 a), b), c) and d) respectively. A carbon fibre tow typically comprises a substantially flat ‘ribbon’ or bundle of fibres, which may be ‘woven’ or otherwise interlinked, wherein the fibres may all comprise carbon, but may equally comprise a mixture of fibres, such as glass, ceramic, Kevlar, or the like, and include one or more carbon fibres therein.
Referring to FIG. 2 of the drawings, if strain sensing is to be performed using the strain/resistance response of the carbon fibres, the fibres or tows 104 is typically insulated each other by means of respective insulative (e.g. glass) layers or ‘blankets’ 106, in order to prevent short circuiting between adjacent fibres or tows. The ends of one or more carbon fibres 104 are made accessible externally of the polymer substrate 102, and electrical contacts 108 are coupled thereto, by means of which strain sensing can be performed. There are many known ways of measuring the resistance of an electrical component, and by way of example only, in some systems, a fixed current is applied to the carbon fibre(s) and the resultant voltage drop across it is measured such that, by means of Ohm's law, the resistance of the fibre(s) can be determined.
However, it has been discovered by the inventors that temperature changes represent a potentially significant source of error in strain measurements due to the thermoresistive response of carbon fibres. For example, the inventors have determined that a change in temperature of +10° C. can result in an apparent strain error of 1500-2000 microstrain (tensile). Typical design strains for CFRP structures are of the order of 2000-6000 microstrain and typical tensile failure strains are of the order of 10,000-15,000 microstrain. It can thus be seen that measurement error caused by temperature fluctuation can result in a significant percentage error in strain measurements in structural materials of this type.
As a result, the use of carbon fibre strain sensors is limited to dynamic measurements on structures with thermal time constants significantly longer than the dynamic response rate of interest, in many practical applications. Indeed, known carbon fibre strain sensors can only provide reliable absolute strain data in cases where the structure is isolated from temperature variations (e.g. laboratory tests). On the other hand, air platforms, for example, are expected to operate across a wide range of temperatures, typically ranging from −55° C. to at least 80° C.
It would, therefore, be desirable to provide a strain sensing arrangement which provides absolute resistance measurements and, therefore, absolute strain data for most practical applications, in a manner that can be integrated with realistic components and structures, and aspects of the present invention seek to address at least some of these issues.